ERRORS IN REFRACTION

For most of us with straight forward refractive errors vision can be corrected to normal. Usually, issues only present if spectacles or contact lenses are not worn for their prescribed purpose.

A refractive error occurs when the shape of your eye does not bent light correctly thereby resulting in a blurred image.

We will now briefly consider,

• Shortsightedness or myopia
• Longsightedness or hypermetropia

And,

• Astigmatism

They will be followed by an eye diagram.

Myopia or shortsightedness

In normal vision parallel rays of light coming from a distance should focus on the retina at the fovea – the central point of the macula. If the eye is too long from back to front, i.e. myopia, then the rays of light are focussed between the lens and the macula and results in blurred distance vision. Spectacles and contact lenses containing concave lenses (minus lenses) can be used to correct this. Near vision is always good provided that the object is brought closer to the eyeball.

In most cases of myopia the lengthening of the eyeball is only moderate and becomes static with skeletal growth and correction should be possible to 6/6. However in a small proportion of myopics the eyeball continues to grow. This condition is known as ‘progressive myopia’ or ‘high myopia’ and normal visual acuity cannot be achieved through the use of spectacles or contact lenses.

The stretching of the retina caused by the abnormal growth can affect the efficiency of the macula and may result in holes or rents in the peripheral or central retina. Since there is a risk of detached retina advice must always be obtained from an ophthalmologist before the child participates in contact sports or activities like diving during swimming or heading a football.

Myopia and accessing the world

• The wearing of prescribed spectacles or contact lenses will be essential for most tasks. Some learners may need encouragement to wear them.
• For near tasks a child or young person may find it easier to work without spectacles and bring work close up to the face
• There may be problems with distance vision; for example, seeing the black/white board
• A book stand may be helpful when developing good posture whilst reading and writing
• All printed materials including diagrams and maps must have good contrast and clear print at an appropriate size. These should be available at the same time as fellow learners without shortsightedness

Hypermetropia (longsightedness)

In this condition the eyeball is too short and the rays of light focus behind the retina resulting in blurred and in some cases non-effective vision. Simple hypermetropia can be corrected to normal vision with convex lenses (+ lenses) so that the light rays are focused onto the fovea. Problems usually occur only when it is associated with other visual conditions such as cataracts. In these cases although spectacles will be prescribed, visual acuity will be reduced. This condition may be associated with a squint.

Hypermtropia and accessing the world

• There is a loss of accommodation, that is, the ability to focus, when objects are brought close to the face
• Long periods of reading or other close tasks should be avoided as they cause discomfort. The learner may complain of visual fatigue or eye strain, headaches  and/or  fluctuating vision
• Some people may have been prescribed low vision devices such as a hand-held magnifier, telescope or a closed circuit television (CCTV). It is essential to provide training and sometimes encouragement to use them

Astigmatism

The main cause of astigmatism is the variation of the refractive power of the cornea or lens due to irregularities in the curvature. This results in a distorted image on the macula. It may be simple to rectify with a cylindrical correction built into the lens of the spectacles, but difficulties can arise when it is associated with myopia and hypermetropia. When it is associated with another visual impairment, correction can be difficult and it can result in reduced visual acuity; that is, how clearly a learner can see detail.

Astigmatism and accessing the world

These are similar to hypermetropia with the addition of,

• A learner may confuse letters and numbers
• Visual fatigue may be a problem after long periods of close visual work or at the end of a school day/week

Ref,

Mason,H. and MaCall,s. 1997. Visual Impairment, Access and Learning for Children and Young People. David Fulton Publishers.